Many sliding stem process control valves are actuated with a control fluid (e.g., air) using well-known diaphragm type or piston type actuators, generally known as sliding stem valves. Sliding stem valves may also be actuated by using a portion of the process fluid flowing through the sliding stem valve, or by using an electric motor actuator. Actuators may be used to automate these control valves by supplying force and motion to open or close a valve. Sliding stem valves have a valve stem (e.g., a sliding stem) that moves a flow control member (e.g., a valve plug) between an open position and a closed position to control a flow of fluid through the valve. An actuator stem operatively couples a linear valve stem to the actuator (e.g., a pneumatic actuator, hydraulic actuator, electric actuator, etc.).
In operation, a control unit supplies the control fluid to the actuator to cause the actuator to position the valve stem or shaft and thus the flow control member to a desired position to regulate fluid flow through the valve. When the valve is closed, the flow control member is typically configured to engage an annular or circumferential seal that encircles the flow path through the valve to prevent fluid flow (e.g., in one or both directions) through the valve.
In process control systems, it may be necessary to override the position of the flow control member to the open position, the closed position, or any other desired position. For example, it may be necessary to open a valve to prevent overpressurization of a vessel or it may be necessary to close a valve to prevent a spill (e.g., a chemical spill) during emergency situations, power failures, or if the control fluid supply to the actuator is shut down. Some known override mechanisms use a hand wheel and screw mounted directly to the actuator so that an operator may manually operate the valve. However, these known override mechanisms only permit manual operation of some valves in one direction and, thus, cannot be used to manually operate the valve in the other direction. Such known manual override mechanisms generally cannot impart more than approximately 2 tons of force on the valve stem due to size and material strength limitations. Additionally, such known manual override mechanisms, such as a hand wheel and screw, must transform rotational energy (from the hand wheel) to linear energy on the valve stem. As a result, the valve stem, or connectors between the valve stem and the actuator stem, may be subject to undesirable rotational and/or shearing forces.